1. Field of the Invention
The present invention generally relates to an injection-locked frequency dividing apparatus, and more particularly, to an injection-locked frequency dividing apparatus with linear mixers.
2. Description of Related Art
Along with the rapid development of communication technologies, the quantity and speed for transmitting data over communication networks have been drastically increased, and broad transmission bands have always been adopted in order to increase the transmission rate. In the wireless communication field, highly integrated and low-cost complementary metal-oxide-semiconductor (CMOS) chips are desired. While in a transceiver of a communication system, a frequency synthesizer is usually adopted for supplying a stable local oscillation signal. Besides, frequency divider is one of the major elements in a frequency synthesizer, and the operation frequency range thereof has to cover the frequency band of the frequency synthesizer.
An injection-locked frequency divider with a wide lock range can be used for designing a broadband frequency synthesizer and eventually applied to a system conforming to different communication protocols, such as IEEE 802.11b/g, 802.16d, and ultra wide band. Thus, an injection-locked frequency divider should be designed to have a wider lock range.
An injection-locked oscillator is usually implemented by using a cross-coupled oscillator as the major element for generating the oscillation signal. When signals having the same phase are injected into such a device with a differential output structure, because the even harmonics of the circuit also have the same phase, they are synchronous to the input signals. Thus, the circuit may have an even divisor when signals having the same phase are input. While designing a divide-by-3 injection-locked frequency divider, the injection-locked effect when signals having the same phase are input is not very ideal for the lock range is very narrow. Thus, a differential input structure has to be adopted.
In a conventional divide-by-3 injection-locked frequency divider, two serial or parallel input transistors are disposed for injecting differential signals from both sides of the circuit, and the injection transistors are served as nonlinear transistor mixers. The nonlinear operations of the transistors generate different harmonic factors in the output, and the nonlinear transistor mixers generate different harmonic and sub-harmonic factors. When an input signal of the divide-by-3 injection-locked frequency divider is mixed to have one third of the input frequency, the core oscillator can be injection-locked. However, the mixed result produced by foregoing circuit is very complicated. Because the power is distributed over a broad frequency domain and there is only a small amount of valid sub-harmonic power, the lock range of the frequency divider is limited and accordingly the application of the frequency divider is affected.